This invention relates generally to video signal processing systems and particularly to NTSC type television video signal processing systems.
As is well known, NTSC television signals are prone to a number of distortions including artifacts such as hanging dots, dot crawl, color contamination, highly visible noise in low frequency signal areas of the video display, a visible line structure due to line interlacing and the loss of vertical and horizontal picture detail because of restricted bandwidth. All of the above have been addressed by the prior art in different ways. For example, line comb filters are often used to derive the video and chroma signal components by adding and subtracting adjacent lines of video. This can eliminate cross contamination of the chroma signal into the luma signal and vice versa and enables a wider bandwidth luma signal to be processed. Comb filtering works well to minimize noise and "crosstalk" except in the presence of video variations which give rise to other well-known problems. In the copending application Ser. No. 349,192, an arrangement for determining when to comb filter and how much to comb filter, i.e. whether and how much to comb the two upper lines of a three line display area, the two lower lines, all three lines or whether to "simple decode" the signal is determined by developing gradients between selected pixels in a spatial area that embraces all three video lines. A processing decision for each pixel is made based upon the developed differences. The final processing decision may be modified by reference to processing decisions made for previous pixels in the spatial area and for decisions that the system indicates should be made for subsequent pixels in the spatial area. With that more precise decision making arrangement, combing is done only when it will be of benefit to the video display and avoided when it may introduce objectionable artifacts or otherwise detract from the video display.
The prior art also discloses many techniques for peaking the video signals, i.e. sharpening the signal transitions to give the impression of a wider bandwidth. Because of the subjective nature of video displays and signal peaking, there are many algorithms for determining when a signal should be peaked and the degree to which it should be peaked. Signal peaking circuits are also frequently used in conjunction with so-called "noise coring" circuits which essentially core the signal, i.e., provide a range of signal levels for which no peaking is performed. The purpose is to avoid emphasizing visible noise in a signal.
The interlaced scanning arrangement of an NTSC signal produces two alternating fields having interleaved lines of video. Consequently, each line of pixels in the display is illuminated only fifty percent of the time, i.e. during one of the two alternating fields. This results in a visible line structure which is disconcerting to viewers. The line structure is especially objectionable in receivers and monitors having large display areas, i.e. large screen and projection type television receivers and video monitors. Many arrangements have been developed for converting an interlaced scan system to a progressive scan system, where each line of pixels is scanned (illuminated) in each field, thus eliminating the visible line structure. This is generally accomplished by scanning the display at twice the normal rate and interspersing interpolated lines of video between each pair of real video lines. Commonly, the interpolated lines of video are developed by averaging the video in each successive pair of video lines or by repeating each real line of video. Copending application Ser. No. 349,192 mentioned above, discloses a progressive scan converter that averages successive lines of real video to develop the interpolated lines of video and also enhances the vertical detail between the real lines of video.
The invention in Ser. No. 550,605 is directed to a progressive scan converter having interpolated pixels based upon spatial gradients that preserve diagonal resolution in the video display. The invention in Ser. No. 549,869 is directed to an adaptive peaking circuit that samples pixels in a spatial area and peaks based upon the signal attitude, i.e. in the direction of greatest indicated change in signal. This invention is directed to a very simple noise coring arrangement for providing symmetrical and asymmetrical peaking and noise coring of the peaking component.